Electron-injection-engineering induced dual-phase MoO2.8F0.2/MoO2.4F0.6 heterostructure for magnesium storage

ABSTRACT Rechargeable magnesium batteries (RMBs) have received increased attention due to their high volumetric capacity and safety. Nevertheless, the sluggish diffusion kinetics of highly polarized Mg2+ in host lattices severely hinders the development of RMBs. Herein, we report an electron injection strategy for modulating the Mo 4d-orbital splitting manner and first fabricate a dual-phase MoO2.8F0.2/MoO2.4F0.6 heterostructure to accelerate Mg2+ diffusion. The electron injection strategy triggers weak Jahn–Teller distortion in MoO6 octahedra and reorganization of the Mo 4d-orbital, leading to a partial phase transition from orthorhombic phase MoO2.8F0.2 to cubic phase MoO2.4F0.6. As a result, the designed heterostructure generates a built-in electric field, simultaneously improving its electronic conductivity and ionic diffusivity by at least one order of magnitude compared to MoO2.8F0.2 and MoO2.4F0.6. Importantly, the assembled MoO2.8F0.2/MoO2.4F0.6//Mg full cell exhibits a remarkable reversible capacity of 172.5 mAh g−1 at 0.1 A g−1, pushing forward the orbital-scale manipulation for high-performance RMBs.


INTRODUCTION
Rechargeable magnesium batteries (RMBs) have great potential for the next-generation energy storage due to low cost, high volumetric capacity (3833 mAh cm −3 ), and dendrite-free formation [1 -3 ].However, strong electrostatic interactions of polarized Mg 2 + and host lattice could cause sluggish electrochemical reaction kinetics in RMBs, which severely restricts their development [4 ,5 ].Thus, seeking advanced cathode materials for realizing rapid ion and charge transfer is highly desirable to alleviate the aforementioned issue.
Recently, molybdenum-based oxides have been widely explored for energy storage applications due to their high theoretical capacity and abundant crystal structure [6 ].Nevertheless, the widespread utilization of molybdenum oxides for RMBs has been restricted by their intrinsic characteristics, such as poor electrical conductivity and sluggish Mg 2 + diffusivity [7 ].Until now, extensive efforts have been devoted to overcome these weaknesses, such as enlarging interlayer spacing by preintercalation molecules or ions [8 ,9 ], shielding the strong polarization of Mg 2 + by introducing H 2 O molecules [10 ], and reducing binding force by doping anions with higher polarizability [11 ].Despite the Mg 2 + migration barriers in metallic oxides in which materials can be significantly reduced, the ionic diffusion kinetics of full cells assembled with a Mg metal anode sti l l remains a challenge in respect to the poor affinity of O 2 − in the crystal frameworks and Mg 2 + .F − doping can significantly reduce both the bandgap value and the Mg-ion migration barriers, which have been proved by theoretical calculation [12 ].In our previous reports, F − substitutional doping of molybdenum-based oxides improved the affinity of anion and Mg 2 + , thereby accelerating Mg 2 + diffusion along three dimensional (3D) pathways in materials [13 ].Unfortunately, the designed electrodes usually lead to an insufficient improvement in magnesium storage due to uncoordinated regulation of the electronic conductivity and ionic diffusivity of materials.
In order to realize the coordination of electronic conductivity and ionic diffusivity, some researchers pay attention to the built-in electric field (BIEF) in the heterostructure materials [14 ,15 ].The BIEF could trigger charge redistribution behavior and regulate the electronic structure for achieving superior conductivity of heterostructure materials [16 ].In addition, the built-in electric field could induce abundant ion adsorption and accelerate ion diffusion from a higher majority concentration to a lower majority concentration in the interfaces of heterostructures [17 ,18 ].However, the strategy of constructing built-in electric fields gives a simple insight into the underlying regulatory mechanisms which limit the improvement of the effect of a BIEF [19 ].In this regard, the deep understanding of constructing a BIEF has been regarded to be a nascent topic [20 ,21 ].Therefore, developing a highly efficient and orbitalscale manipulation strategy to achieve the establishment of a BIEF is urgent but sti l l remains challenging.
In this work, we propose an electron injection strategy to induce Mo 4d-orbital splitting manner modulation and fabricate an orthorhombic/cubic phase MoO 2.8 F 0.2 /MoO 2.4 F 0.6 (o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 ) heterostructure to improve Mg storage performance.The electron injection strategy triggers a weak Jahn-Teller distortion in MoO 6 octahedra and reorganization of the Mo 4d orbitals, leading to a partial phase transition (orthorhombic to cubic) for achieving a dual-phase MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure.The designed heterostructure possesses an abundant built-in electric field, which simultaneously enhances electron transfer and ion diffusion in the crystal frameworks.The Mott-Schottky analysis and X-ray absorption near-edge structure (XANES) analyses are employed to understand the generation and effect mechanism of the BIEF in the heterostructure.In addition, I-V tests, galvanostatic intermittent titration techniques (GITTs) and density functional theory (DFT) calculations were investigated to test the enhancement of the electronic conductivity and ionic diffusivity.As a result, the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode displays high reversible capacity (303.8 mAh g −1 at 0.1 A g −1 ) and excellent rate performance (154.1 mAh g −1 at 2 A g −1 ).Importantly, the assembled o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //Mg ful l cel l exhibits a remarkable reversible capacity of 172.5 mAh g −1 at 0.1 A g −1 .Briefly, our current work provides an efficient strategy for achieving coordinated regulation of electronic conductivity and ionic diffusivity in cathode materials.

RESULTS AND DISCUSSION
Oxygen (O) and fluorine (F) are adjacently located in the same period (Period II) of the Periodical Table of Elements, which were selected to form the octahedral TMX 6 (where 'TM' is 'transition metal' , 'X' is 'O or F') with molybdenum for the study [22 ].However, the difference in electronegativity and valence electron configuration (O: 2s 2 2p 4 , F: 2s 2 2p 5 ) results in the injection of electrons and reorganization of the Mo 4d orbitals in the octahedral TMX 6 when replacing the O element with an F element (Fig. 1 S1).Besides, the characteristic peaks of c-MoO 2.4 F 0.6 can be found at 837.6 and 948.4 cm −1 .In addition, two characteristic peaks of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure appear at 813.1 and 845.7 cm −1 , corresponding to the Mo = X symmetric stretching vibrations of the o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 materials.Two characteristic peaks of the heterostructure appear at 991.3 and 1002.7 cm −1 , which signifies the Mo = X asymmetric stretching vibrations of the o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 materials [25 ].Interestingly, compared with o-MoO 2.8 F 0.2 and c -MoO 2.4 F 0.6 , the peaks of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure show a slight shift, which is related to the change in the bond angles/lengths of the MoX 6 octahedron during the different number of electrons injected ( Fig. S22) [26 ].In addition, the XRD pattern and Raman spectrum of MoO 3 are displayed in Fig. S23.The chemical valence states of samples are explored by X-ray photoelectron spectroscopy (XPS).In Fig. S24 heterostructure, the Mo 3d spectrum can be fitted to three pairs of peaks that are related to Mo 6 + (236.0 and 233.0 eV), Mo 5 + (234.6 and 231.5 eV), and Mo 4 + (233.5 and 229.9 eV) [27 ].Remarkably, the binding energy of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure is apparently higher by ∼0.1-0.3 eV than that in c-MoO 2.4 F 0.6 , while the binding energy of the heterostructure is slightly lower compared to o-MoO 2.8 F 0.2 .The opposite shift of Mo 3d confirms the distribution of opposite charges through the interface in o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures [28 ].In Fig. S25, the F 1s XPS spectrum of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure is also red-shifted and blue-shifted compared to the o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 , respectively, which is associated with the charge transfer between o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 .As a contrast, XPS spectra of pure MoO 3 are displayed in Fig. S26.Furthermore, we have performed electron paramagnetic resonance (EPR) spectra to identify the existence of molybdenum vacancies in o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 materials.As shown in Fig. S27, compared with the pristine MoO 3 sample, the o-MoO 2.8 F 0.2 sample exhibits an intense EPR signal at g = 2.002 which could be attributed to the presence of unpaired electrons in Mo species following minor F − doping.Furthermore, the characteristic peak of the Mo −O/F dangling bond for the c-MoO 2.4 F 0.6 sample is much stronger than that of the o-MoO 2.8 F 0.2 sample, demonstrating a significantly increased concentration of molybdenum defects following major F − doping.These cationic vacancies are conducive to unlocking the inactive basal plane of the layered crystal structure and triggering shallow impurity levels in the energy band, giving rise to fast Mg 2 + diffusion and electron transport during charge/discharge processes.Moreover, the Mo K-edge X-ray absorption near-edge structure (XANES) analyses of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure was implemented (Fig. 2 g).The distinct pre-edge peak of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure exhibits an asymmetrical structure, which may be ascribed to the hybridization of orbitals [29 ].The first-derivative of the XANES spectra is shown in Fig. S28.The Fourier-transform (FT) and wavelettransform (WT) EXAFS spectra (Fig. 2 h, i) confirm the existence of both Mo-O/F and Mo-Mo coordination [30 ].The peaks of Mo-Mo in Mo foil are concentrated at 2.47 Å, and the peaks of Mo-O/F and Mo-Mo in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure correspond to 1.63 Å and 3.21 Å, respectively [31 ].
To gain more detailed information about the band gap (E g ), valence band (E vb ), and conductive band (E cb ) levels of samples, related tests are Pre-equilibrium e - e -e - e - e - e - e -e -  conducted and shown in Fig. 3 .The Boltzmann distribution and Gauss' law reveals the electron distributions in the space charge region, which are associated with the BIEF at the interface of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure [32 ].Therefore, the Mott-Schottky equation can be obtained through Poisson's equation [33 ]: where C , A , e , V , k B and T are the interfacial capacitance area, charge, voltage, Boltzmann's constant, and absolute temperature, respectively.In Fig. 3 S30).Theoretically, the valence band (E vb ) is relevant to the band gap (E g ) and conductive band (E cb ), which can be described as [36 ]: Thereby, the E vb of o-MoO  S2.Accordingly, significant charge transfer from c-MoO 2.4 F 0.6 to o-MoO 2.8 F 0.2 and imbalanced charge distribution are further displayed in the calculated results (Fig. 3 c).Once o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 are in intimate contact, the charge redistribution wi l l spontaneously occur near the interfaces until their Fermi levels reach equilibrium (Fig. 3 d and e).As a result, the electrons and holes wi l l be accumulated on the o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 sides, respectively, which forms a certain built-in electric field for providing a continuous electron flow.As proof, the electrical conductivity of the o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures are displayed, and their calculated conductivities are 7.2 × 10 −3 , 9.8 × 10 −3 , and 2.1 × 10 −2 S cm −1 , respectively (Fig. 3 f).The result reveals that the BIEF in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure significantly enhances conductivity by orders of magnitude.To deeper understand the role of heterojunctions in enhancing electronic conductivity and ionic diffusivity, the total density of states (TDOS) calculations and the finite element simulation were carried out.As shown in Fig. 3 g-i, the total density of states (TDOS) of o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures reveal that F − doping related states have emerged in the bandgap of those materials.Notably, the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure displays a narrow band gap in the vicinity of the Fermi level (E f ), which generates more unpaired electrons and electronic conduction owing to the built-in electric field in the heterostructure.
To clarify the merits of the heterostructure, the obtained materials were evaluated in a magnesium half-cell, in which the activated carbon cloth (AC) with high surface area and abundant active sites are used as counter electrodes.The AC electrode delivers reversible capacities of 0.14 mAh at 0.1 A g −1 in the window between −0.15 and 0.15 V vs.A g/A gCl ( Fig. S33), which can provide enough capacity and charges to balance the Mg 2 + intercalation process in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode [37 ,38 ].In the half-cell, the magnesium ions derived from the electrolyte are inserted/extracted in the cathode and both anions (TFSI − ) and cations (Mg 2 + ) from the electrolyte are reversibly adsorbed/desorbed in the activated carbon cloth (AC) anode.The anions adsorb while the cations desorb on the surface of the AC anode during the discharge process, and vice versa during the charge process ( Fig. S34).The reaction process of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //AC cells can be formulated as follows: In the discharge process: Anode : AC containing y Mg 2+ surface In the charge process: In addition, the charge/discharge processes of the MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //AC cells are schematical ly i l lustrated in Fig. S35.Cyclic voltammetry (CV) curves of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrodes are measured for the initial cycles at a scan rate of 0.2 mV s −1 .As depicted in Fig. S36, a   the Mg 2 + diffusion behaviors, the Nyquist plots and fitted results of electrodes are shown in Fig. 4 c.The o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode exhibits smaller Warburg coefficients ( σ = 20.28)than that of the o-MoO 2.8 F 0.2 electrode ( σ = 38.89)and c-MoO 2.4 F 0.6 electrode ( σ = 51.47)[39 ,40 ], which further proves the efficient diffusion kinetics of Mg 2 + in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode.Moreover, the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode delivers a reversible capacity of 303.8 mAh g −1 at 0.1 A g −1 , which is much higher than that of pure o-MoO 2.8 F 0.2 (249.3 mAh g −1 ) and c-MoO 2.4 F 0.6 (100.6 mAh g −1 ) (Fig. 4 d).Therefore, Mg 2 + prefers to insert/extract in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 , which may be attributed to the abundant phase boundary.Furthermore, the ion diffusion coefficient (D) of electrodes was investigated by GITT tests.The D of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode is 9.12 × 10 −13 to 1.99 × 10 −10 cm 2 s −1 , which is much higher than pure o-MoO 2.8 F 0.2 electrode and c-MoO 2.4 F 0.6 electrode (Fig. 4e; Figs S37, S38, and S39), revealing the efficient ion transfer enabled by the dual-phase heterostructure.In addition, the D values of the MoO 3 electrode is 2.27 × 10 −14 to 2.5 × 10 −12 cm 2 s −1 ( Fig. S40), which is much lower than o-MoO 2.8 F 0.2 electrode (1.41 × 10 −13 to 4.25 × 10 −12 cm 2 s −1 ) and c-MoO 2.4 F 0.6 electrode (5.55 × 10 −14 to 4.15 × 10 −12 cm 2 s −1 ), showing that the substitution of fluorine for oxygen could activate low-valent redox-active transition metals or create more cationic vacancies.
Further investigation of the correlations between the phase ratios of MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure and electrochemical performance are shown in Fig. S41.Although the cation defects in the materials significantly improve the Mg 2 + diffusion, the interfacical content in the MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure plays an even more crucial role in accelerating ion diffusion kinetics.Therefore, the D of MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -1 and MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -2 electrodes ranges from 8.94 × 10 −13 to 2.32 × 10 −11 cm 2 s −1 , which is notably lower compared to the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode (4.58 × 10 −13 to 1.48 × 10 −11 cm 2 s −1 ), indicating sluggish ion diffusion kinetics in the other phase ratios.In addition, the MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -1 and MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -2 electrodes exhibit a low capacity and poor cycling stability.The D values of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode is superior to the majority of previously reported molybdenum-based oxide electrodes ( Table S3).The CV curves of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode at different scan rates are displayed in Fig. S42.The b-values of the oxidation/reduction peaks are assessed at 0.88 and 0.86, respectively, demonstrating the electrochemical storage behavior contributed by both capacitive and diffusion behavior.To prove the enhancement of the cathode-electrolyte compatibility of the heterostructure in magnesium ful l cel ls, we assembled o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //Mg, MoO 2.8 F 0.2 //Mg, and MoO 2.4 F 0.6 //Mg full cells.In magnesium full cells, the MTB electrolyte was used due to its ability to achieve reversible deposition and dissolution of magnesium ( Fig. S43).Impressively, the GCD curves of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //Mg full cells were performed at 0.1 A g −1 ( Fig. S44), which displays relatively little polarization and high reversible capacity of 172.5 mAh g −1 (Fig. 4 f).The low initial Coulombic efficiency of a ful l cel l is attributed to the thermodynamic instability of a Mg/organic electrolyte interface and the electrochemical conditioning process of MTB electrolyte during the initial Mg 2 + deposition/dissolution process.In Fig. 4 g, the rate performance of the cell was determined at different current densities and the corresponding reversible capacities are 203.3,132.1, 73.6, 50.3, and 35.9 mAh g −1 , respectively.Furthermore, a reversible capacity of 194.1 mAh g −1 was easily restored at 0.05 A g −1 , indicating the favorable rate tolerance capabi lity of the cel l.As a contrast, the cycling performances and rate performances of o-MoO 2.8 F 0.2 //Mg and c-MoO 2.4 F 0.6 //Mg are displayed in Figs S45 and S46, which are inferior to the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //Mg cell.In addition, the magnesium storage property of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 //Mg cell is also superior to the majority of previously reported molybdenumbased materials ( Table S4).In addition, the simple Mg(TFSI) 2 /AN and Mg(TFSI) 2 /DME electrolyte was used in the magnesium full cells system ( Figs S47 and S48).However, these electrolytes can not support the operation of magnesium full cells due to the Mg metal passivating in polar aprotic solvents and the incomplete dissociation of magnesium salts.
To further evaluate the kinetic in depth, in situ EIS measurement and the corresponding detailed fitting data were employed at various charge/discharge states of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode (Fig. 5 a).When gradually discharging, the electrode shows a continual decrease of resistances due to the formation of Mo 4 + and rich contact of the heterointerface during the Mg 2 + insertion process.When gradually charging, the continual increase of resistances is attributed to the reversible Mg 2 + extraction process and exhibits excellent interfacial stability within the whole cycle.As shown in Fig. S49, STEM-EDS mapping images display the uniform dispersion of the Mg element, which reveals the Mg 2 + insertion during the discharge process.In contrast, only the fuzzy Mg element outline can be observed within the charged o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode.In addition, ex-situ Mo 3d XPS spectra were performed to examine the valence state of Mo in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode during the charge/discharge process (Fig. 5 b).After full discharging to 0.01 V, the Mo 4 + content is significantly improved, which derives from the reduction of Mo 6 + and Mo 5 + .After full charging to 2.0 V, parts of Mo 4 + in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode are returned to Mo 6 + and Mo 5 + , confirming the reversible evolution of the Mo valence state during the discharge/charge process.Besides, when discharged to 0.1 V, a sharp Mg 1s peak is observed, which indicates the surface adsorption and intercalation of Mg 2 + in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode (Fig. 5 c).When charged to 2.0 V, the Mg signals almost disappeared, implying the reversible insertion/extraction of Mg 2 + .As depicted in Fig. S50, the peaks of Mo 6 + , Mo 5 + , and Mo 4 + remain almost unchanged after 100 cycles, suggesting the high stability of the cell.In addition, the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure shows the layered and uniform nanostructure as well as the morphology are consistent with the SEM observation before 100 cycles ( Fig. S51), which indicates the excellent stability of these materials.Moreover, the time-of-flight secondary-ion mass spectrometry (TOF-SIMS) of o-MoO   ) and an excellent rate performance (154.1 mAh g −1 at 2 A g −1 ).Even after being assembled with a Mg anode, the ful l cel l can provide a high specific capacitance of 172.5 mAh g −1 at 0.1 A g −1 , exhibiting great potential for practical application.This work would offer a meaningful insight to simultaneously improve charge transfer and ion diffusion in the crystal frameworks for superior magnesium storage.
pair of obvious redox reaction peaks are attributed to the insertion and extraction of Mg 2 + in the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode.These peaks are well overlapped over the three cycles, demonstrating the good electrochemical reversibility of the heterostructure.In Fig.4a, the galvanostatic charge-discharge (GCD) curves of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode were performed at various current densities, which displays welldefined voltage platforms.Compared with pure o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 electrodes, the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode delivers improved reversible capacities of 321.7, 295.9, 258, 202.1, and 154.1 mAh g −1 at the current density 0.1, 0.2, 0.5, 1, and 2 A g −1 , respectively (Fig.4 b).Noteworthy, the relatively high Coulombic efficiency of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure is attributed to the enhancement of electronic conductivity and ionic diffusivity by at least an order of magnitude.To better understand the impact of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure on
2.4 F 0.6 heterostructure match well with the orthorhombic phase MoO 2.8 F 0.2 (Cmcm (63), JCPDS No. 25-0563) and cubic phase MoO 2.4 F 0.6 (Pm-3 m (221), JCPDS No. 24-0770), which verifies the coexistence of o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 in the heterostructure[23 ,24 ].The as-synthesized o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure has been analyzed by X-ray diffraction (XRD) patterns and their Rietveld refinement ( Fig.S21), in which the ratio of the orthorhombic phase MoO 2.8 F 0.2 to the cubic phase MoO 2.4 F 0.6 in the heterostructure is 1:1.In addition, the ratio of o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 as 8:2 (denoted by MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -1) and 2:8 (denoted by MoO 2.8 F 0.2 /MoO 2.4 F 0.6 -2) have been prepared to understand the correlations between phase ratios and electrochemical performance.The Raman spectra of o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures are displayed in Fig. 2 e.The peaks at 804.3 and 1005.4 cm −1 represent the Mo = X stretching vibrations and X −Mo −X stretching vibrations (where X represents O or F) of o-MoO 2.8 F 0.2 materials, respectively ( Table , the survey spectra indicate the existence of Mo, O, and F elements in o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.From the Mo 3d spectra (Fig. 2 f), the peaks at 236.1 and 233.1 eV are assigned to Mo 3d 3/2 and Mo 3d 5/2 signals of Mo 6 + , and the peaks at 234.7 and 231.6 eV are attributed to Mo 3d 3/2 and Mo 3d 5/2 signals of Mo 5 + .In addition, the Mo 3d XPS spectrum of c-MoO 2.4 F 0.6 shows extra peaks at 233.4 and 229.8 eV corresponding to Mo 3d 3/2 and Mo 3d 5/2 signals of Mo 4 + , which originate from the successful accomplishment of the major F − doping.In the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.(d and e) Schematic diagrams of bond alignment of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.(f) Electrical conductivity of o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.Calculated TDOS of (g) o-MoO 2.8 F 0.2 , (h) c-MoO 2.4 F 0.6 , and (i) o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.
a and Fig. S29, the Mott-Schottky plots of o-MoO 2.8 F 0.2 and c-MoO 2.
[34 ].6 derived from the plots of Log Z against potential exhibit a positive slope, which is a typical n-type semiconductor[34 ].In addition, the flat band potentials (E fb ) of materials can be confirmed from the Mott-Schottky curves.The E fb of o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 are 0.23 and 0.19 eV vs.A g/A gCl, which are equal to 0.43 and 0.39 eV vs. normal hydrogen electrode (NHE), respectively.Notably, as an n-type semiconductor, 2.8 F 0.2 and c-MoO 2.4 F 0.6 can be calculated as 2.02 and 1.75 eV, respectively.Besides, the calculated work functions ( φ) of o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 are 4.67 and 4.59 eV, respectively ( Figs S31, and S32).The E vb , E g , and E cb of o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 are summarized in Table 2.8 F 0.2 , c-MoO 2.4 F 0.6 and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 cathodes are implemented to explore the stability of high electronic conductivity o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 in chloride-containing electrolyte systems ( Fig. S52).The presence of CH 2 O − and Cl − ionic fragments is correlated with the decomposition of both the solvent and chloride in the electrolyte.With the prolonged sputtering time, the signal intensity of CH 2 O − and Cl − ionic fragments decreases and stabilizes at the same time in o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrodes, which means that the high electronic conductivity o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 could not worsen the continuous breakdown of the MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode at different voltage states.(h) The relative concentrations of magnesium ions in o-MoO 2.8 F 0.2 and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures.MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode during the discharge/charge process.In addition, the heterostructure maintains structural stability during battery operation without irreversible phase transitions and significant volume changes.The ex-situ XRD patterns and variations of lattice volume of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 at different voltage states were performed based on an AC anode (Fig. 5 f, g).During the discharge and charging process, the (020) and (040) diffraction peaks of o-MoO 2.8 F 0.2 shift and return, and the characteristic peaks of c-MoO 2.4 F 0.6 show almost no change, indicating a highly reversible Mg 2 + insertion/extraction process of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode.To gain insights into the intrinsic properties of phases affecting ionic transfer kinetics, we conducted investigations into Mg 2 + migration pathways within the orthorhombic and cubic phase structures using bond valence site energy (BVSE) calculations ( Fig. S55).Visibly, the orthorhombic phase MoO 2.8 F 0.2 provides intermittent and narrow zigzag paths for Mg 2 + migration along the ac plane directions.Differently, the cubic phase MoO 2.4 F 0.6 possesses successive and broad Mg 2 + migration pathways along the 3D (a, b, and caxis) directions.Impressively, the designed o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure could trigger the reversible ion insertion/extraction and accelerate ion diffusion for achieving superior magnesium storage.To further prove the Mg 2 + diffusion kinetics of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure, the finite element simulation of Mg 2 + concentration distribution for o-MoO 2.8 F 0.2 and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructures were simulated and analyzed ( Fig. S56), in which the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure has a more uniform Mg 2 + concentration and faster Mg 2 + flux distribution compared to the o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 .Figure 5 h clearly and prominently displays the curves of relative concentration of magnesium ions in the structural materials, which suggests that the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure can achieve fast and efficient ion diffusion during the charge and discharge process.Besides, we summarized the comparison of the o-MoO 2.8 F 0.2 , c-MoO 2.4 F 0.6 , and o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrodes during the Mg 2 + insertion/extraction process.CONCLUSIONS To summarize, we innovatively designed the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure by an electron injection strategy to simultaneously enhance electronic conductiv ity and ionic diffusiv ity in RMBs.The electron injection strategy induces weak Jahn-Teller distortion in MoO 6 octahedra and Mo 4d-orbital splitting, leading to a partial phase transition from orthorhombic MoO 2.8 F 0.2 to cubic MoO 2.4 F 0.6 .The tailored dual-phase MoO 2.8 F 0.2 /MoO 2.4 F 0.6 heterostructure triggers the built-in electric field, which shortens ion diffusion length (L) and activates ion diffusivity (D) in the crystal frameworks to reduce Mg 2 + diffusion time in two aspects (t ≈ L 2 /D).In addition, the BIEF induces charge redistribution to decrease the band gap of the electronic structure.As a result, compared to pure o-MoO 2.8 F 0.2 and c-MoO 2.4 F 0.6 , the electronic conductivity and ionic diffusion coefficient of o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 is improved by at least one order of magnitude.Impressively, the electrode demonstrates a respectable reversible capacity (303.8 mAh g −1 at 0.1 A g −1 Figure 5. (a) The in-situ Nyquist plots and the corresponding fitting parameter (R ct ) of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6 electrode at various discharged/charged states.The ex-situ (b) Mo 3d XPS spectra, (c) Mg 1s XPS spectra, (d) normalized Mo K-edge XANES spectra, (e) FT-EXAFS spectra of the cell at discharged and charged states.(f) The GCD profiles and ex-situ XRD patterns, and (g) the volume changes of o-c 2 /MoO 2.4 F 0.6 electrode during magnesiation/demagnesiation, the Mo K-edge XANES analyses were measured.During the discharge process, the Mo edge shifts toward lower energy with the slight reduction of Mo 6 + and Mo 5 + , and the white-line intensity decreases with the lattice expansion of the o-c MoO 2.8 F 0.2 /MoO 2.4 F 0.6